Back-ported the patch of the mysql-5.6 code line that

fixed several defects in the greedy optimization:

1) The greedy optimizer calculated the 'compare-cost' (CPU-cost)
   for iterating over the partial plan result at each level in
   the query plan as 'record_count / (double) TIME_FOR_COMPARE'

   This cost was only used locally for 'best' calculation at each
   level, and *not* accumulated into the total cost for the query plan.

   This fix added the 'CPU-cost' of processing 'current_record_count'
   records at each level to 'current_read_time' *before* it is used as
   'accumulated cost' argument to recursive 
   best_extension_by_limited_search() calls. This ensured that the
   cost of a huge join-fanout early in the QEP was correctly
   reflected in the cost of the final QEP.

   To get identical cost for a 'best' optimized query and a
   straight_join with the same join order, the same change was also
   applied to optimize_straight_join() and get_partial_join_cost()

2) Furthermore to get equal cost for 'best' optimized query and a
   straight_join the new code substrcated the same '0.001' in
   optimize_straight_join() as it had been already done in
   best_extension_by_limited_search()

3) When best_extension_by_limited_search() aggregated the 'best' plan a
   plan was 'best' by the check :

   'if ((search_depth == 1) || (current_read_time < join->best_read))'

   The term '(search_depth == 1' incorrectly caused a new best plan to be
   collected whenever the specified 'search_depth' was reached - even if
   this partial query plan was more expensive than what we had already
   found.

1 files changed, 5 insertions, 5 deletions

diff --git a/mysql-test/r/explain.result b/mysql-test/r/explain.result
index 52c58f088a1..e08e3e22307 100644
--- a/mysql-test/r/explain.result
+++ b/mysql-test/r/explain.result
@@ -196,13 +196,13 @@ create table t2 (dt datetime not null);
 insert into t1 values ('2001-01-01 1:1:1', '1:1:1'),
 ('2001-01-01 1:1:1', '1:1:1');
 insert into t2 values ('2001-01-01 1:1:1'), ('2001-01-01 1:1:1');
-SET @save_optimizer_switch=@@optimizer_switch;
-SET optimizer_switch='semijoin_with_cache=off';
+SET @save_join_cache_level=@@join_cache_level;
+SET join_cache_level=0;
 flush tables;
 EXPLAIN SELECT OUTR.dt FROM t1 AS OUTR WHERE OUTR.dt IN (SELECT INNR.dt FROM t2 AS INNR WHERE OUTR.dt IS NULL );
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	OUTR	ALL	NULL	NULL	NULL	NULL	2	Using where
-1	PRIMARY	INNR	ALL	NULL	NULL	NULL	NULL	2	Using where; Start temporary; End temporary
+1	PRIMARY	INNR	ALL	NULL	NULL	NULL	NULL	2	Using where; FirstMatch(OUTR)
 flush tables;
 SELECT OUTR.dt FROM t1 AS OUTR WHERE OUTR.dt IN (SELECT INNR.dt FROM t2 AS INNR WHERE OUTR.dt IS NULL );
 dt
@@ -210,13 +210,13 @@ flush tables;
 EXPLAIN SELECT OUTR.dt FROM t1 AS OUTR WHERE OUTR.dt IN ( SELECT INNR.dt FROM t2 AS INNR WHERE OUTR.t < '2005-11-13 7:41:31' );
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	OUTR	ALL	NULL	NULL	NULL	NULL	2	Using where
-1	PRIMARY	INNR	ALL	NULL	NULL	NULL	NULL	2	Using where; Start temporary; End temporary
+1	PRIMARY	INNR	ALL	NULL	NULL	NULL	NULL	2	Using where; FirstMatch(OUTR)
 flush tables;
 SELECT OUTR.dt FROM t1 AS OUTR WHERE OUTR.dt IN ( SELECT INNR.dt FROM t2 AS INNR WHERE OUTR.t < '2005-11-13 7:41:31' );
 dt
 2001-01-01 01:01:01
 2001-01-01 01:01:01
-SET optimizer_switch=@save_optimizer_switch;
+SET join_cache_level=@save_join_cache_level;
 drop tables t1, t2;
 #
 # Bug#47669: Query showed by EXPLAIN EXTENDED gives different result from original query